The embodiments of this invention will be discussed following a description of related prior art in connection with FIGS. 1 and 2 of the accompanying drawings.
FIG. 1 is a schematic circuit diagram showing a conventional Schmitt trigger circuit which functions as a voltage comparator and comprises two transistors Q1 and Q2, and three resistors R1, R2 and R3. In operation, when the voltage of an input signal applied to an input terminal 2 is initially lower than a first comparator level V.sub.1 (FIG. 2), the transistors Q1 and Q2 are off and on, respectively. As a result, a low output voltage V.sub.L is derived from an output terminal 4. If the input signal rises up to the first comparator level V.sub.1, the transistors Q1 and Q2 change their states becoming on and off, respectively. The circuit of FIG. 1, therefore, produces a high output voltage V.sub.H at the output terminal 4. The first comparator level V.sub.1 is expressed: EQU V.sub.1 =(V.sub.CC -V.sub.CE(sat))R3/(R2+R3)+V.sub.BE ( 1)
where
V.sub.CC : voltage of power supply PA1 V.sub.CE(sat) : collector saturation voltage of Q2 PA1 V.sub.BE : emitter-base voltage of Q1
Conversely, when the input signal, after exceeding V.sub.1, falls to a second comparator level V.sub.2 (FIG. 2), the Schmitt trigger circuit of FIG. 1 returns to its original state producing the low voltage V.sub.L at the output terminal 4. The second comparator level V.sub.2 is given by following equation; EQU V.sub.2 =(V.sub.CC -V'.sub.CE(sat))R3/(R1+R3)+V.sub.BE, (2)
where V'.sub.CE : collector saturation voltage of Q1. Assuming that V'.sub.CE(sat) is identical with V.sub.CE(sat), a hysteresis range V.sub.HYS is obtained from the equations (1) and (2): ##EQU1##
It will therefore be apparent from the equations (1) and (2) that since V.sub.BE is a temperature dependent parameter, the first and the second comparator levels V.sub.1 and V.sub.2 are adversely affected by ambient temperature change. Therefore, there has been a problem with the conventional Schmitt trigger circuit that a temperature compensating means should be provided in order to assure stable comparator levels against ambient temperature change.
It is therefore a primary object of this invention to provide an improved transistor circuit having two different comparator levels which can be determined or designed with ease.
Another object of this invention is to provide an improved transistor circuit having two different comparator levels which are free from ambient temperature change.